Embodiments disclosed herein relate generally to an apparatus and method of delivering a fluid mixture into a wellbore and recapture/recycling of an output CO2.
Hydraulic fracturing, commonly known as hydro fracturing, or simply fracturing, is a technique used to release petroleum, natural gas or other substances for extraction from underground reservoir rock formations. A wellbore is drilled into the reservoir rock formation, and a treatment fluid is pumped which causes fractures and allows for the release of trapped substances produced from these subterranean natural reservoirs. Current wellhead fracturing systems utilize a process wherein a slurry of fracturing fluid and proppant (e.g. sand) is created and then pumped into the well at high pressure. When water-based fracturing fluids are used, a process referred to as hydro fracturing, the proppant, water and appropriate chemicals can be mixed at atmospheric pressure and then pumped up to a higher pressure for injection into the well. However, if fluids other than water (e.g. liquid CO2 or liquid propane) are used as the fracturing fluid, then these fluids must be kept at a sufficient pressure throughout the hydraulic fracturing system to avoid undesired vaporization. As a result, the blending of these fluids with proppant, chemicals, etc. must also be accomplished while the fluids are kept under a sufficiently high pressure.
CO2 fracturing, a water-free fracturing technique, avoids many of the environmental problems associated with hydro fracturing such as soil contamination due to top-side fluid spills and use of clean drinking water sources. In addition, hydrocarbon production can be improved through reduced damage to the formation and proppant pack, yet several factors limit commercial application. Such factors include cost of CO2, availability of CO2, flaring of CO2 and effective proppant transport to name a few. CO2 as a fracturing fluid must be injected at the well site as a supercritical liquid. Typically, CO2 fracturing operations provide that the CO2 is delivered from an external source, stored on site and blended with proppant under pressure. Current CO2 fracturing processes utilize pressurized proppant blending and storage of the amount of proppant required to complete a single fracturing stage under pressure to support blending, which limits both proppant and CO2 storage capacities. During clean-up and flow-back of the well, the CO2 is typically vented/flared to the atmosphere.
Known pressurized blenders capable of blending vaporizing fracturing fluids, such as CO2, with the proppant at a suitably high pressure utilize a pressurized proppant storage vessel arrangement to feed and meter the proppant into the pressurized fracturing fluid. These known lock-hopper based pressurized blenders require pre-loading with the proppant to be utilized during a given fracture stage. The pressurized proppant storage vessels used typically have a capacity in the range of approximately 20-40 tons of proppant (e.g., sand). The limited volume capacity of the proppant storage vessel system provides for limited amounts of proppant to be blended with the CO2 fracturing fluid. In addition, these known pressurized blenders require an undesirably long elapsed time to reload them with proppant for the next fracture stage. In some instances, some pressurized blender operations require the blender unit be moved off-site to another location for the purpose of reloading with proppant, also requiring an undesirably long time and potentially adding to the truck traffic associated with fracturing operations. In many instances, the limited capacity requires specialized logistics and on-pad (or off-pad) proppant handling equipment to be used in conjunction with the proppant storage vessel based pressurized blenders.
As a result of the limited capacity of the proppant under pressure, injection rates and the volume of an output flow of CO2/proppant slurry are limited since blender operation has to be periodically stopped to allow for refilling of proppant storage and/or supplying of CO2. This stoppage in operation results in lost man-hours, or a larger number of blenders on the wellpad, either of which increases costs.
Accordingly, there is a need for an improved CO2 fracturing system and method for delivering fracturing fluid into a wellbore that will enable the blending and pumping of essentially unlimited quantities of proppant and fracturing fluid to form the fluid mixture. The ability to deliver unlimited quantities will provide for continuous operation of the system, enable fracture plans to be based upon reservoir stimulation requirements without imposing equipment constraints, and therefore providing overall a more efficient system.